The present invention relates to automatic transmissions and in particular to control of flows into an accumulator.
Automatic transmissions include a number of cooperating mechanical and fluid mechanical elements to achieve a desired goal. The input and output of the automatic transmissions are coupled by a number of in-series planetary gear sets. The planetary gear sets reside inside drums and the mechanical ratio through a planetary gear set changes depending on whether the drum is free to rotate, or held fixed. Gear selection of automatic transmissions is made by selectively tightening and releasing bands residing around drums.
The bands are tightened on the drums by hydraulically operated band servos receiving automatic transmission fluid under pressure. If the servo operates too quickly, the bands stop the drum too quickly, the transmission may shift harshly, possibly damaging parts, and providing an unpleasant sensation to occupants of the vehicle. In order to prevent this event, automatic transmissions include accumulators. An accumulator is a device hydraulically connected to the flow of automatic transmission fluid to the servo. The accumulator includes a piston or equivalent device which moves in response to a change in pressure of the automatic transmission fluid to dampen changes in pressure. For example, the accumulator may be a simple cylinder and piston with springs biasing the piston to a neutral position in the cylinder. When an abrupt change in pressure occurs in the flow of automatic transmission fluid to the servo, the piston moves in the accumulator to dampen the response of the servo, and thereby reduce the harshness of the shift. Unfortunately, different drivers and different uses of vehicles are not always suited to the degree of damping provided by the accumulator. A simple inexpensive solution is not available and as a result, drivers must live with undesirable shift characteristics because no simple and inexpensive means are available to adjust the damping.
Additionally, the accumulator is also in fluid communication with an Electronic Pressure Control (EPC) accumulator valve which provides an opposing transmission fluid flow into the accumulator to provide faster or firmer shifting when desired. Unfortunately the flow through the EPC accumulator valve is not moderated and in some instances results in an abrupt change in pressure to the band servos causing harsh shifting.